The material and shape of the current conductor or current feeder of electrical lamps having a glass bulb quite substantially determine the manufacture, function and quality of such lamps. The term "lamps" especially comprises halogen filament lamps and discharge lamps such as mercury vapor high-pressure lamps, halogen-metal vapor lamps, and xenon-HP-discharge lamps.
Much attention has been paid in the past to this technical field. Electrical conductors for feeding current in lamps with or without gas filling are, as a rule, fused in quartz glass or squeezed into the latter. Molybdenum, owing to its high melting point and its favorable coefficient of thermal expansion as compared to glass, has been found to be a suitable conductor material for feeding current.
Other material requirements a molybdenum conductor is expected to satisfy are ductility, good moldability, high mechanical strength, resistance to oxidation or corrosion, especially versus halides, and fusibility with other components of the conductor.
The problem of DE-C-29 47 230 is to make available an electric lamp with a current feeder of superior mechanical strength, and this patent proposes that the molybdenum foil used heretofore be replaced by a molybdenum foil in which yttrium oxide particles are dispersed in the molybdenum in an amount of 0.25 to 1% of the weight of the molybdenum. However measured against current requirements, this material has inadequate corrosion resistance and especially insufficient oxidation properties.
DE-C-30 06 846 proposes coating of the electrical conductor for the current feed--the latter being a molybdenum or a tungsten foil--with a second metal selected from the group of tantalum, niobium, vanadium, chromium, zirconium, titanium, lanthanum, scandium and hafnium. According to this patent, the molybdenum or tungsten conductor can be coated by using vapor deposition, cathode atomization, electrolysis and other methods.
Such a coating process, however, is complicated and cost intensive, and, if carried out in an economical way, does not assure an evenly thick application in a way such that the desired protection against corrosion is assured in all areas of the coating. Furthermore, current feeders coated in this way lack sufficient fusibility particularly when chromium is used as the coating material. It may be necessary to first fuse the basic material with another component part before it is coated.
U.S. Pat. No. 5,021,711 relates to the protection of molybdenum foils against oxidation as well, such foils being used as conductors in vacuum lamps. It proposes that the molybdenum foil be refined superficially by means of ion implantation, using chromium, aluminum, or combinations of said metals. Adequate fusibility is lacking in this case as well, and this process is complicated and expensive for this type of material. Furthermore, the manufacturing costs of the mass produced quartz lamps are increased by this process to an unsatisfactory extent.
Furthermore, EP-B-0 309 749 deals with increasing the resistance of molybdenum to oxidation, such molybdenum being intended for use in electric lamps as an electrical conductor within the sealing zone. An enhanced useful life of the material, especially in a corrosive environment at elevated temperatures from 250.degree. to 600.degree. C., is to be accomplished by coating the basic molybdenum material with an alkali metal silicate.
Drawbacks of an electrical conductor produced in this manner include its high manufacturing cost and high brittleness or susceptibility to breakage of such components. Again due to the lack of adequate fusibility, it is necessary to "go the expensive way", i.e., first fusing and then coating.
For increasing the resistance to oxidation and also the fusibility and resistance to media containing hydrogen, EP-B-0 098 858 proposes coating of the molybdenum current feeder with a layer of rhenium. Rhenium is an expensive material. The known methods for producing such a coating are costly, which means in this case too, the main drawback again is inadequate economy of the electrical conductors treated in such a way.
A molybdenum alloy used for electrical conductors as the current feeder in lamps is known from AT-B 395 493, such alloy consisting of 0.01 to 5% by weight of one or several oxides of the lanthanides, the balance being Mo.
As compared to other known conductor materials, this material does in fact offer excellent fusibility and resistance to high temperatures; however, other application-specific material features are less favorable than those of individual, previously known Mo-alloys. The sum of all properties of this material recommends its use as a wire-like current feeder fused in hard glass, but not as a strip or foil fused in quartz glass.
In addition to the selection of the material, processing of the latter when it is fused as an electrical conductor in lamp glass is of special importance. For example, a special process is known from EP-A 0 311 308, by which the metal and the glass are fused with each other in a special way within the sealing zone in the presence of a hydrogen/nitrogen gas mixture. However, this method and others cannot in any case satisfactorily compensate the known problems posed when using such materials as electrical conductors.
Finally, according to EP-B-0 275 580, it is proposed to manufacture current feeder wires for lamps from an alloy substantially consisting of molybdenum, 0.01 to 2% by weight yttrium oxide, and 0.01 to 0.8% by weight molybdenum boride. This alloy was intended as an improvement over a potassium-silicon-doped molybdenum alloy; however, it does not offer any improvements versus a pure molybdenum-yttrium oxide alloy, especially not with respect to the resistance of the latter to oxidation. A serious drawback of this material is that it frequently causes socket cracks in the glass within the zone where it is fused or squeezed in, such cracking being caused by changes in the strength of the material in the course of recrystallization of the latter in the fusing step.
JP-B-85058296 describes a heat-resistant alloy consisting of 10 to 70% by weight yttrium oxide and/or cerium oxide, the balance molybdenum, which is used for protective tubes for thermoelements. This publication does not disclose anything about any suitability of the alloy for current feeders in lamps.